Variant reported 1685, 1737 and 1862

According to Muir's Historical Atlas, Heligoland was not Danish until 1714. Perhaps the red, white and blue striped flags relate to a pre-1714 Frisian period? (...) The date given for the chart on which the "Holygoland"flag appears in Wilson 1986 p. 68 is 1685-6. The dates 1737 and 1862 seem to have come from somewhere else.
David Prothero, 27 Sep 2000

Variant reported 1716

Civil Flag reported 1848

2:3 image by Jorge Candeias

Horizontal tricolor green-red-white. In use as local flag until the present.
Norman Martin, Mar 1998

Regarding Helgoland:
It is not clear to me where do the red-green-white colours come from. Before the British invasion 1807 and later takeover 1814, the only Helgoland flags appear to have been multi-striped flags in blue, white and red. So where did the green originate and when was the current flag established? It was already being used by 1848, when it appeared on a flag chart.
Santiago Dotor, 27 Sep 2000

In his letter of 11th May 1888, quoted here, the Governor also referred to, "an old Frisian motto:- Green is the grass, red the cliff and white the sand, these three make the colours of
Heligoland".
David Prothero, 27 Sep 2000

Westermann Lexicon der Geographie 1969 has: "Settled by Frisians; it belonged to the Duchy of Schleswig from 1402 till 1714; from that date till 1807 it was Danish, and from
1807-1890 British. It was exchanged for Zanzibar and the Caprivi strip.".
Jarig Bakker, 28 Sep 2000

The German editors of Norie and Hobbs 1971 added two charts (which were not originally in Norie and Hobbs 1848) with German flags that were important over time. One of them is no. 2, Helgoland ab 1696 (vorher ohne Wappen), i.e. Heligoland from 1696 (before without arms): as the above image, with the arms from no. 107 mirrored.
Peter Hans van der Muijzenberg, 12 Nov 2001

Heligoland was British for some decades in the 19th Century but was given to Germany, in exchange for which the Germans gave up their claim on Uganda. Actually in exchange in exchange
for commercial rights in Zanzibar (Zanzibar Treaty, 1890).
It was also occupied for some years more than the rest of West Germany, but unlike Saar I think it was considered a part of the Federal Republic from the foundation. It belongs to the state of Lower Saxony (Niedersachsen)
I used to think it belonged to Schleswig-Holstein. At least it was Schleswig's up to 1814, but also has a flag of its own.
Santiago Dotor, 1 Oct 2003

Hel(i)goland does indeed belong to Schleswig-Holstein (part of Landkreis Pinneberg).
Helgoland was under British occupation from 1945 to 1952. The islanders were evicted, and the island was used as a bombing target. In 1947, the British tried (unsuccessfully) to blow up the island entirely. After many protests, bombing stopped in 1952, and the islanders were allowed to return.
Stefan Schwoon, 2 Oct 2003

curious flag of Helgoland (c.1914)

image by Jarig Bakker, 18 Jun 2005

In the "Vlaggenalbum", edited by Droste's Chocolate factory in Haarlem, March 1914, is a curious flag of Helgoland: three horizontal stripes green - red- white, with a square red canton, charged with a white castle
of three towers.
I'm not sure whether the canton refers to Hamburg, but I can't imagine what else it could be...
Jarig Bakker, 18 Jun 2005